The global tendency toward high development reveals the problem of energy shortage that is urged to be resolved. In contrast to the exhaustion of petroleum anticipated in the near future, solar energy is deemed to replace petroleum. One way converting solar energy into electricity is to utilize a semiconductor material having a specific band gap to form a p-n junction. The carriers (holes/electrons) on the lateral sides of the p-n junction absorb photons having energy larger than that of the specific band gap, and then combine together to generate current. Solar cell made of polycrystalline silicon is popular presently although its converting efficiency is still low, about 10% to 15%. Because polycrystalline silicon is easy to be obtained and the cost thereof is low, it is still utilized and popular extensively. Because the band gap of silicon is 1.12 eV, it can absorb only a portion of the range of wavelength within the infrared in the spectrum. To increase the power, the area of the solar panel must be increased as well, thus the application is inconvenient.
Multi-junction tandem solar cell structure is acknowledged as a structure with the highest converting efficiency. For example, the tandem structure of Ge/GaAs/GaInP is triple junctions whose crystal lattices are matching with each other. The top cell made of GaxIn1-xP(1.85 eV; x˜0.5) has larger band gap which can receive photons of higher energy within the range of wavelength from ultraviolet to visible light. A middle cell made of GaAs has band gap of 1.42 eV, which can receive photons whose wavelength within the range close to infrared. A bottom cell made of Ge has lower band gap of 0.74 eV, which can receive photons from the light passing the first two cells and having wavelength within the range of infrared. Because the range of the solar radiation spectrum the structure receives is more extensive, the converting efficiency is increased to over 30%.
The converting efficiency of tandem solar cell depends on a lot of factors. For example, there are options about the materials of the tandem by different band gaps for receiving more extensive solar radiation spectrum, optimization of thickness of each layer for tuning short-circuit current (Isc) or open-circuit voltage (Voc) of the solar cell for increasing power, or extent of the epitaxy lattice-match of each layer to reduce the lattice-defects for increasing the converting efficiency.
Owing to the lattice constant of the material itself, when the lattice-mismatch between a substrate and a material of a lower layer occurs, the film growing on the substrate can result in lattice-defects and inferior quality. The usual way to solve lattice-mismatch in multi-junction tandem solar cell is to add a transparent buffer layer between the two layers mismatching with each other. For example, there is a research that a double-junction of the GaAs/GaInAs series forms a solar cell structure of a combination of 1.42 eV/1.1 eV to replace the known GaAs/Ge cell structure. Because the lattice constant of the GayIn1-yAs having band gap of 1.1 eV(y˜0.8) is 5.75 Å mismatching to the that of GaAs and the extent of lattice-mismatch is still as high as 1.8%, another usual way is to grow a transparent graded layer between the GaAs cell and GaInAs series cell. The material of the transparent graded layer can be GaxIn1-xP series. The lattice constant of GaxIn1-xP can be increased from 5.65 Å (x˜0.5) to 5.75 Å (x˜0.25) by increasing the content of In gradually. Therefore, one surface of the graded layer of GaInP series matches with the GaAs cell and another surface thereof matches with the GaInAs cell to solve the problem of lattice-match.
For higher converting efficiency, a multi-junction tandem solar cell structure having more junctions makes it more difficult to choose the materials Thus, the present invention provides a bonding structure of an optoelectronical semiconductor device to offer another solution to solve the problem of connecting two different materials.